If attaching an image file that it's filename contains specific characters to Confluence page, then it's not displayed as image correctly.
The cause is that image wiki markup character '!' is escaped with backslash. Please see my attached screenshots.

For instance:

Image filename	Wiki Markup	After saving page	Result
rs1.png	!rs1.png|border=1!	'!' is not escaped.
rs①.png	!rs①.png|border=1!	'!' is escpaed.

'①' means U+2460 character code, which is 'Circled Digit One'.

I hope this bug will be fixed. However first I want to know which unicode character is escaped with backslash after saving page.
By solving its mechanism, I can understand unavailable characters to image filename and provide Japanese users it as the limitation.